A load sensing circuit is adopted as a hydraulic circuit of a working machine. The load sensing circuit is intended to supply a hydraulic fluid to a cylinder in full measure in a circuit, the circuit measuring not only operating lever positions but also volumes of hydraulic pressure in the circuit, and thereafter calculating a cylinder load pressure taking a margin pressure into account to control pump flow rates to be supplied to the circuit based on the calculated value (For example, refer to Patent Document 1). Although, when a large load is applied on the cylinder, and an engine revolution speed drops below that at which an engine can output horsepower most efficiently, an electronic control system controls to reduce the pump flow rate and to maintain the engine revolution speed, whatever the case may be, through a flow rate control mainly with the load sensing circuit, the engine is operated to activate a target actuator by a fixed lever stroke without stalling regardless of the load.
By the way, in some working machines, a regeneration circuit for returning hydraulic fluid from a rod side of a boom cylinder to a head side thereof is used in order to achieve an increase in actuator speeds and a saving in pump flow rates when a boom is being laid down (For example, refer to the Patent Document 2). In this regeneration circuit, when a cylinder head pressure is higher than a rod pressure while the cylinder is moving in its contracting direction, a regeneration to return the hydraulic fluid from the head side to the rod side is performed, the cylinder contracting movement comes close to its end, and the regeneration is complete when the rod pressure becomes higher than the head pressure.
Patent Document 1: Japanese Utility Model Application Laid-open No. 6-84005 (Paragraphs 0001, 0002, FIGS. 2 and 3)
Patent Document 2: Japanese Patent Application Laid-Open 2006-9888 (Paragraph 0002)